Below are listed typical fields of study.
In many cases, you can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s HD. A successful check allows you to remember a bit of useful information about that monster.
For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.
Action: Usually none. In most cases, making a Knowledge check doesn’t take an action—you simply know the answer or you don’t.
Try Again: No. The check represents what you know, and thinking about a topic a second time doesn’t let you know something that you never learned in the first place.
Synergy: If you have 5 or more ranks in Knowledge (arcana), you get a +2 bonus on Spellcraft checks.
If you have 5 or more ranks in Knowledge (architecture and engineering), you get a +2 bonus on Search checks made to find secret doors or hidden compartments.
If you have 5 or more ranks in Knowledge (geography), you get a +2 bonus on Survival checks made to keep from getting lost or to avoid natural hazards.
If you have 5 or more ranks in Knowledge (history), you get a +2 bonus on bardic knowledge checks.
If you have 5 or more ranks in Knowledge (local), you get a +2 bonus on Gather Information checks.
If you have 5 or more ranks in Knowledge (nature), you get a +2 bonus on Survival checks made in aboveground natural environments (aquatic, desert, forest, hill, marsh, mountains, or plains).
If you have 5 or more ranks in Knowledge (nobility and royalty), you get a +2 bonus on Diplomacy checks.
If you have 5 or more ranks in Knowledge (religion), you get a +2 bonus on turning checks against undead.
If you have 5 or more ranks in Knowledge (the planes), you get a +2 bonus on Survival checks made while on other planes.
If you have 5 or more ranks in Knowledge (dungeoneering), you get a +2 bonus on Survival checks made while underground.
If you have 5 or more ranks in Survival, you get a +2 bonus on Knowledge (nature) checks.
Untrained: An untrained Knowledge check is simply an Intelligence check. Without actual training, you know only common knowledge (DC 10 or lower).